Chlamydia (kluh-MID-e-uh) trachomatis (truh-KOH-muh-tis) is a common sexually transmitted infection (STI) caused by bacteria. You might not know you have chlamydia because many people don't have signs or symptoms, such as genital pain and discharge from the vagina or penis.

Chlamydia trachomatis affects mostly young women, but it can occur in both men and women and in all age groups. It's not difficult to treat, but if left untreated it can lead to more-serious health problems.

Early-stage Chlamydia trachomatis infections often cause few or no signs and symptoms. Even when signs and symptoms occur, they're often mild, making them easy to overlook.

Signs and symptoms of Chlamydia trachomatis infection can include:

• Painful urination
• Vaginal discharge in women
• Discharge from the penis in men
• Painful sexual intercourse in women
• Bleeding between periods and after sex in women
• Testicular pain in men

Chlamydia trachomatis can also infect the rectum, either with no signs or symptoms or with rectal pain, discharge or bleeding. You also can get chlamydial eye infections (conjunctivitis) through contact with infected body fluids.

When to see a doctor

See your doctor if you have a discharge from your vagina, penis or rectum, or if you have pain during urination. Also, see your doctor if you learn your sexual partner has chlamydia. Your doctor will likely prescribe an antibiotic even if you have no symptoms.

The Chlamydia trachomatis bacterium is most commonly spread through vaginal, oral and anal sex. It's also possible for pregnant women to spread chlamydia to their children during delivery, causing pneumonia or a serious eye infection in the newborns.

Factors that increase your risk of Chlamydia trachomatis include:

• Being sexually active before age 25
• Having multiple sex partners
• Not using a condom consistently
• History of sexually transmitted infection

Chlamydia trachomatis can be associated with:

• Pelvic inflammatory disease (PID). PID is an infection of the uterus and fallopian tubes that causes pelvic pain and fever. Severe infections might require hospitalization for intravenous antibiotics. PID can damage the fallopian tubes, ovaries and uterus, including the cervix.
• Infection near the testicles (epididymitis). A chlamydia infection can inflame the coiled tube located beside each testicle (epididymis). The infection can result in fever, scrotal pain and swelling.
• Prostate gland infection. Rarely, the chlamydia organism can spread to a man's prostate gland. Prostatitis can cause pain during or after sex, fever and chills, painful urination, and lower back pain.
• Infections in newborns. The chlamydia infection can pass from the vaginal canal to your child during delivery, causing pneumonia or a serious eye infection.
• Ectopic pregnancy. This occurs when a fertilized egg implants and grows outside of the uterus, usually in a fallopian tube. The pregnancy needs to be removed to prevent life-threatening complications, such as a burst tube. A chlamydia infection increases this risk.
• Infertility. Chlamydia infections — even those that produce no signs or symptoms — can cause scarring and obstruction in the fallopian tubes, which might make women infertile.
• Reactive arthritis. People who have Chlamydia trachomatis are at higher risk of developing reactive arthritis, also known as Reiter's syndrome. This condition typically affects the joints, eyes and urethra — the tube that carries urine from your bladder to outside of your body.

The surest way to prevent chlamydia infection is to abstain from sexual activities. Short of that, you can:

• Use condoms. Use a male latex condom or a female polyurethane condom during each sexual contact. Condoms used properly during every sexual encounter reduce but don't eliminate the risk of infection.
• Limit your number of sex partners. Having multiple sex partners puts you at a high risk of contracting chlamydia and other sexually transmitted infections.
• Get regular screenings. If you're sexually active, particularly if you have multiple partners, talk with your doctor about how often you should be screened for chlamydia and other sexually transmitted infections.
• Avoid douching. Douching decreases the number of good bacteria in the vagina, which can increase the risk of infection.

The Centers for Disease Control and Prevention recommends chlamydia screening for:

• Sexually active women age 25 or younger. The rate of chlamydia infection is highest in this group, so a yearly screening test is recommended. Even if you've been tested in the past year, get tested when you have a new sex partner.
• Pregnant women. You should be tested for chlamydia during your first prenatal exam. If you have a high risk of infection — from changing sex partners or because your regular partner might be infected — get tested again later in your pregnancy.
• Women and men at high risk. People who have multiple sex partners, who don't always use a condom or men who have sex with men should consider frequent chlamydia screening. Other markers of high risk are current infection with another sexually transmitted infection and possible exposure to an STI through an infected partner.

Screening and diagnosis of chlamydia is relatively simple. Tests include:

• A urine test. A sample of your urine is analyzed in the laboratory for presence of this infection.

• A swab. For women, your doctor takes a swab of the discharge from your cervix for culture or antigen testing for chlamydia. This can be done during a routine Pap test. Some women prefer to swab their vaginas themselves, which has been shown to be as diagnostic as doctor-obtained swabs.

  For men, your doctor inserts a slim swab into the end of your penis to get a sample from the urethra. In some cases, your doctor will swab the anus.

If you've been treated for an initial chlamydia infection, you should be retested in about three months.

Chlamydia trachomatis is treated with antibiotics. You might receive a one-time dose, or you might need to take the medication daily or multiple times a day for five to 10 days.

In most cases, the infection resolves within one to two weeks. During that time, you should abstain from sex. Your sexual partner or partners also need treatment even if they have no signs or symptoms. Otherwise, the infection can be passed back and forth between sexual partners.

Having chlamydia or having been treated for it in the past doesn't prevent you from getting it again.

If you think you have a sexually transmitted infection, such as Chlamydia trachomatis, see your family doctor.

What you can do

Before your appointment, prepare to answer the following questions:

• When did your symptoms begin?
• Does anything make them better or worse?
• What medications and supplements do you take regularly?

You also might want to prepare a list of questions to ask your doctor. Sample questions include:

• Should I be tested for other sexually transmitted infections?
• Should my partner be tested or treated for chlamydia infection?
• Should I avoid sex during treatment? How long should I wait?
• How can I prevent chlamydia infection in the future?

What to expect from your doctor

Your doctor is likely to ask you a number of questions, such as:

• Do you have a new sexual partner or multiple partners?
• Do you use condoms consistently?
• Do you have pelvic pain?
• Do you have pain while urinating?
• Do you have sores or unusual discharge?